Group III nitride semiconductor crystals such as aluminum nitride, gallium nitride and indium nitride have a wide range of band gap energy and the band gap energy values of these substances are about 6.2 eV, about 3.4 eV and about 0.7 eV, respectively. A mixed crystal semiconductor having any composition can be formed from these group III nitride semiconductors, thereby making it possible to obtain a band gap energy value between the above band gap energy values according to the composition of the mixed crystal.
Therefore, it is theoretically possible to manufacture a light emitting device which emits a wide range of light from infrared light to ultraviolet light by using the group III nitride semiconductor crystals. Particularly, the development of a light emitting device using an aluminum-based group III nitride semiconductor (mainly aluminum gallium nitride mixed crystal) is now under way energetically. Short-wavelength light of an ultraviolet range can be emitted by using an aluminum-based group III nitride semiconductor, and light emitting sources such as an ultraviolet light emitting diode for white light sources, an ultraviolet light emitting diode for sterilization, a laser which can be used to read and write a high-density optical disk memory and a communication laser can be manufactured.
A light emitting device using an aluminum-based group III nitride semiconductor (also called “aluminum-based group III nitride semiconductor light emitting device”) can be manufactured by forming semiconductor single crystal thin films as thick as several microns (more specifically, thin films which serve as a p type semiconductor layer, a light emitting layer and an n type semiconductor layer) sequentially on a substrate like a conventional semiconductor light emitting device. The formation of the semiconductor single crystal thin films can be carried out by a crystal growth process such as molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE). The formation of a preferred multi-layer structure for the aluminum-based group III nitride semiconductor light emitting device by using the above process is now under study.
Currently, the substrate for use in ultraviolet light emitting devices is generally a sapphire substrate from the viewpoints of crystal quality as a substrate, ultraviolet light transmission, mass-productivity and cost. However, when the sapphire substrate is used, problems arise due to differences in physical properties between the sapphire substrate and aluminum gallium nitride forming a semiconductor multi-layer film.
For example, a crystal defect called “misfit dislocation” is introduced into a semiconductor multi-layer film due to a difference in lattice constant between the substrate and the semiconductor multi-layer film. Further, as there is a difference in thermal expansion coefficient between a growth layer and the substrate, the growth layer or the substrate cracks or warps due to the difference in thermal expansion coefficient while the temperature is reduced from the temperature for forming the semiconductor multi-layer film in the manufacture of the semiconductor multi-layer film. The above dislocation or crack reduces the light emitting performance of the semiconductor multi-layer film.
To solve these problems, various film forming conditions and structures have been contrived for the semiconductor multi-layer film. However, to solve the above problems substantially, it is ideal to use a substrate having a lattice constant close to that of the semiconductor multi-layer film and a small difference in thermal expansion coefficient from that of the semiconductor multi-layer film. It can be said that an aluminum-based group III nitride single crystal substrate, that is, an aluminum nitride single crystal substrate or an aluminum gallium nitride single crystal substrate is the most suitable substrate. However, an aluminum-based group III nitride single crystal substrate having a large area and homogeneity cannot be manufactured stably at present. If a high-quality aluminum-based group III nitride single crystal substrate can be manufactured, it solves the above problems and expected to contribute to the improvement of light emitting performance and the implementation of an ultraviolet light source.
Since an aluminum-based group III nitride single crystal must have a thickness of at least 100 μm to be used as a substrate, it is not practical to manufacture an aluminum-based group III nitride single crystal substrate using the above crystal growth process having a slow film forming speed.
As an aluminum-based group III nitride single crystal growing method having a fast film forming speed, there is known hydride vapor phase epitaxy (HVPE) (refer to JP-A 2003-303774, JP-A 2006-073578 and JP-A 2006-114845). Since it is difficult to control the film thickness accurately in HVPE as compared with MBE and MOVPE, HVPE is not suitable for the formation of a crystal layer for semiconductor light emitting devices but capable of obtaining a single crystal having high crystallinity at a fast film forming speed. Therefore, an aluminum-based group III nitride single crystal substrate which could not be obtained by conventional MBE and MOVPE can be mass-produced at a practical level by HVPE.
To grow an aluminum-based group III nitride crystal by HVPE, for example, a vapor-phase epitaxial apparatus shown in FIG. 1 is used. The apparatus shown in FIG. 1 comprises a reactor body composed of a cylindrical quartz glass growth chamber (reaction tube) 11, heating means 12 arranged outside the growth chamber 11 and a substrate support table (also simply called “susceptor” hereinafter) 13 installed in the growth chamber 11 and has a structure that carrier gases and a raw material gases are supplied from one end portion of the growth chamber 11 and the carrier gases and an unreacted gas are discharged from the other end portion.
The growth chamber on the raw material gas feed side has a concentric triple tube nozzle structure that a triple tube is inserted into a predetermined area from the end portion, a group III halide gas containing aluminum halide which is a group III element source gas diluted with a carrier gas (for example, a hydrogen gas) is supplied through the inside space of the inner tube 15 of the triple tube, a nitrogen source gas (for example, an ammonia gas) diluted with a carrier gas (for example, a hydrogen gas) is supplied through the space between the intermediate tube 16 and the outer tube 17 of the triple tube, and a barrier gas (for example, a nitrogen gas) is supplied through the space between the inner tube 15 and the intermediate tube 16. The barrier gas is supplied to prevent the blockage of a gas exhaust port by a product which is formed right after the group III element source gas and the nitrogen source gas are mixed together and reacted with each other and deposited near the gas exhaust port.
The substrate (for example, a sapphire substrate) 14 is placed on the susceptor 13 and heated by the heating means to grow an aluminum-based group III nitride through a reaction between the group III element source gas and the nitrogen source gas on the substrate 14.
The substrate is generally heated at 1,000° C. or higher to obtain a good crystal layer (growth layer). To heat the substrate, there are (1) a method in which the growth chamber is directly heated with a heater provided outside the growth chamber to heat the substrate by heat conduction or radiation heat through the wall of the reactor, (2) a method in which a cylindrical heat generating member (for example, carbon) provided in the growth chamber is heated by high-frequency heating from the outside of the growth chamber to heat the substrate by its radiation heat, (3) a method in which the substrate is held on a carbon susceptor and the susceptor is heated by high-frequency heating from the outside of the growth chamber to heat the substrate by its heat conduction, (4) a method in which light is applied from the outside of the growth chamber to heat the substrate, (5) a method in which a heating element is buried in the susceptor and electricity is applied to heat the susceptor so as to heat the substrate by its heat conduction, and (6) a method in which the susceptor and/or the substrate are/is heated by an electromagnetic wave such as microwave.
When an aluminum-based group III nitride is produced by HVPE using the conventional vapor-phase epitaxial apparatus shown in FIG. 1, the yield is less than 10%. Therefore, it cannot be said that productivity is always satisfactory. The term “yield” as used herein means the group III metal ratio (wt %) of the aluminum-based group III nitride immobilized on the substrate to the supplied group III raw material gas.
As a method in which a halide gas and an ammonia gas as the nitrogen source gas are used like the method of the present invention, there is known one in which aluminum nitride particles are formed from an aluminum trichloride gas and an ammonia gas by external heating system at a maximum temperature of about 1,100° C. (refer to the Journal of American Ceramics Society 77 [8] 2009-2016, (1994)). In the method of the present invention, the substrate is heated up to 2,000° C. by limiting the gas supply temperature and the pipe heating temperature at the time of carrying the gases more precisely to form bulk single crystals or polycrystals on the substrate. Therefore, this method basically differs from the method of forming solid powders simply by reacting raw material gases.